The present invention relates to a gate circuit for a gate turn-off thyristor (hereinafter referred to as a "GTO"), and more particularly to a gate circuit in which power loss is extremely low.
Various gate circuits for GTO's have been published. A typical one of these gate circuits includes a turn-on circuit and a turn-off circuit each connected between the gate and cathode of a GTO. The turn-on circuit is formed of a series combination of a turn-on power source (namely, a power source for turning on the GTO) and a transistor, and the turn-off circuit is formed of a series combination of a turn-off power source (namely, a power source for turning off the GTO) and another transistor. When the GTO is turned on, the transistor in the turn-on circuit is made conductive, and the transistor in the turn-off circuit is made nonconductive. On the other hand, when the GTO is turned off, the transistor in the turn-on circuit is made nonconductive and the transistor in the turn-off circuit is made conductive.
Such a gate circuit causes no trouble when the anode current of the GTO is small. However, when the anode current of the GTO becomes large, it is required to cause a fairly large current to flow in order to turn off the GTO, since the turn-off gain of the GTO is small, that is, has a value of 3 to 5. Accordingly, the transistor in the turn-off current is required to have a large current capacity. A transistor having a large current capacity is expensive, and a gate circuit including such a transistor is uneconomical.
In order to solve this problem, for example, the following circuit arrangement is disclosed in U.S. Pat. No. 4,117,350, issued on Sept. 26, 1978. That is, the transistor in the turn-off circuit and a thyristor are connected in parallel, so that a major portion of an initial peak gate current flows through this thyristor and a predetermined limited portion of the gate current flows through the transistor in the turn-off circuit. By using such a circuit arrangement, the turn-off circuit can be formed of a transistor and a thyristor which are relatively inexpensive, and therefore an economical gate circuit can be formed.
Methods of supplying a turn-on signal and a turn-off signal to such a gate circuit have been provided. For example, one of the methods is disclosed in U.S. Pat. No. 4,115,707, issued Sept. 19, 1978. In this method, a turn-on signal is outputted from a control signal source, and a turn-off signal is formed by a one-shot circuit which is operated in response to the stoppage of the turn-on signal. However, in order to reduce power loss in the gate circuit, it is required to make a further appropriate improvement in the gate circuit.